Rails - execution sequence of after create callback & nested attributes

Question

I have a simple set up of User and UserProfile model with User has_one :user_profile and UserProfile belongs_to :user.

But I am unable to wrap my head around how Rails defines execution order of after_create callback and accepts_nested_attributes_for defined in my model. Lets consider these two cases.

Case 1:

class User < ActiveRecord::Base
  has_one :user_profile
  accepts_nested_attributes_for :user_profile
  after_create :test_test
end

Now, if I create a user(with user_profile_attributes hash too) via the console, the after_create callback is triggered after the user and its user profile is created.

Case 2: If the after_create is placed at the top,

class User < ActiveRecord::Base
  after_create :test_test
  has_one :user_profile
  accepts_nested_attributes_for :user_profile
end

the callback is triggered after a user has been created but before creating a user profile.

Is this the way it is expected to function. What does Rails do internally here? Is the execution sequence simply determined by the order of the code?

Where do I start to dig deeper into or debug this ?

Answer 1

The order of the declarations in your model can have an impact on the execution order of the code. This is a source for various weird things. (for example, currently callback definitions and has_and_belongs_to_many associations are order dependent: https://github.com/rails/rails/pull/8674 )

To debug the issue you need to browse the rails source. Since your problem has to do with execution order, callbacks and nested attributes I would start by reading up on:

• https://github.com/rails/rails/blob/master/activerecord/lib/active_record/nested_attributes.rb#L256
• https://github.com/rails/rails/blob/master/activerecord/lib/active_record/callbacks.rb#L302
• https://github.com/rails/rails/blob/master/activemodel/lib/active_model/callbacks.rb#L98

This gives you the necessary background to dig deeper. You'll notice that accepts_nested_attributes_for calls into add_autosave_association_callbacks https://github.com/rails/rails/blob/master/activerecord/lib/active_record/autosave_association.rb#L173 This method adds an after_create callback and as far as I know callbacks are executed in order of definition.